Eyepiece optical system for a single lens reflex camera

ABSTRACT

An eyepiece optical system, which is used in a single lens reflex camera having a penta mirror, includes a negative first single lens element, a positive second single lens element, and a negative third single lens element, in this order from the penta mirror; wherein the second single lens element is a diopter adjusting lens element which is adjustable along the optical axis; and the following conditions are satisfied: 
     
       
         −2&lt;f 1 /f e &lt;0  (1) 
       
     
     
       
         0.2&lt;f 2 /f e &lt;0.5  (2) 
       
     
     
       
         −10&lt;f 3 /f e &lt;−1  (3) 
       
     
     wherein 
     f 1  designates the focal length of the first single lens elements; 
     f 2  designates the focal length of the second single lens element; 
     f 3  designates the focal length of the third single lens element; and 
     f e  designates the focal length of the entire eyepiece optical system when the diopter is −1.0.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an eyepiece optical system for a singlelens reflex camera.

2. Description of the Related Art

In a single lens reflex camera, an erecting optical system using ahollow penta mirror instead of a penta prism is well known. Since apenta mirror has the reflection-optical-path in the air, the equivalentair thickness along the optical axis cannot be made shorter up to 1/ntimes (n: the refractive index of a penta prism to be compared) of theequivalent air thickness of the penta prism. Conversely, since thereflection-optical-path is ‘n’ times as long as that of the penta prismhaving the same geometrical distance of the reflection-optical-path, thefocal length of an eyepiece optical system is made longer, and a findermagnification is lowered. Accordingly, a special design is required forthe eyepiece optical system. For example, Japanese Unexamined PatentPublication Nos. 9-146019, and United States Patent PublicationNo.5,136,427 propose a two-lens-element eyepiece optical systemconstituted by a positive lens element and a negative lens element, inthis order from the side of a penta mirror.

However, the eyepiece optical system in Japanese Unexamined PatentPublication No. 9-146019 is for a single lens reflex camera which uses afilm type smaller than a 135-type film, more concretely, for an APS(advanced photo system) camera. Therefore the eyepiece optical systemcannot be converted to an eyepiece optical system for a single lensreflex camera in which the a 135-type film is used. Furthermore, theeyepiece optical systems in U.S. Pat. No. U.S. Pat. No. 5,136,427 is notprovided with an optical system having a diopter adjusting function. Ofcourse it is theoretically possible to change the diopter by moving atleast a portion of lens elements constituting an eyepiece opticalsystem. However, since an optical system having a diopter adjustingfunction is not originally considered, if an attempt is made to satisfya condition for maintaining a sufficient eye relief (the distance fromthe final surface, at the side of a photographer's eye, of the eyepieceoptical system to the eyepoint) with respect to the light rays from theperiphery of the field-of-view, a range of adjustment is smaller even ifthe lens diameter is increased, whereby an effect on a diopteradjustment is insufficient.

Furthermore, U.S. Pat. No. 5,313,327 proposes an eyepiece optical systemfor a penta mirror, the eyepiece optical system of which includes apositive single lens element. Also, this eyepiece optical system cantheoretically change the diopter by moving the positive single lenselement. However, since the amount of change in the diopter is smallwith respect to the amount of movement of the positive single lenselement, a sufficient range of diopter adjustment cannot be obtained,and even in such a range, a sufficient eye relief with respect to thelight rays from the periphery of the field-of-view cannot be maintained.

Moreover, in any of the above mentioned eyepiece optical systems, thechange in the apparent visual angle upon the adjustment of the diopterhas not been considered. The adjusting of the diopter is usuallyperformed while a photographer is looking through the finder, andaccordingly, is unpleasant for the photographer to observe the apparentvisual angle being varied. In particular, when the diopter is suitablyset for the photographer, if the apparent visual angle is narrowed, thefeel of unpleasantness may be intensified.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an eyepiece opticalsystem for a single lens reflex camera in which a 135-type film is used,and a penta mirror is provided as an erecting optical system; throughwhich a diopter adjustment can be performed. Moreover, even when thediopter adjustment is being performed, change in the apparent visualangle is small. Furthermore, it is another object to provide a smallersized eyepiece optical system in which a penta mirror is employed, and asufficient eye relief with respect to the light rays from the peripheryof the field-of-view can be maintained.

In order to achieve the above mentioned objects, there is provided aneyepiece optical system for a single lens reflex camera, which utilizesa penta mirror. The eyepiece optical system includes a negative firstsingle lens element, a positive second single lens element, and anegative third single lens element, in this order from the penta mirror;wherein the second single lens element is a diopter adjustment lenselement which is adjustable along the optical axis direction; and thefollowing conditions are satisfied:

−2<f_(i)/f_(e)<0  (1)

0.2<f₂/f_(e)<0.5  (2)

−10<f₃/f_(e)<−1  (3)

wherein

f₁ designates the focal length of the first single lens elements;

f₂ designates the focal length of the second single lens element;

f₃ designates the focal length of the third single lens element; and

f_(e) designates the focal length of the entire eyepiece optical systemwhen the diopter is −1.0.

The eyepiece optical system preferably satisfies the followingcondition:

0.02<d₃/(n₃×f_(e))<0.07  (4)

wherein

d₃ designates the thickness of the third single lens element; and

n₃ designates the refractive index of the third single lens element withrespect to the d-line.

The eyepiece optical system preferably satisfies the followingcondition:

−1.0<SF1<0  (5)

wherein

SF1 designates a value defined by (r_(1s)+r_(1e))/(r_(1s)−r_(1e));

r_(1s) designates the radius of curvature of the penta-mirror-sidesurface of the first single lens element; and

r_(1e) designates the radius of curvature of the photographer's-eye-sidesurface of the first single lens element.

The third single lens element preferably includes a negative meniscuslens element having a convex surface on the penta mirror side. Moreconcretely, the third single lens element preferably satisfies thefollowing condition:

3<SF3<12  (6)

wherein

SF3 designates a value defined by (r_(3s)+r_(3e))/(r_(3s)−r_(3e));

r_(3s) designates the radius of curvature of the penta-mirror-sidesurface of the third single lens element; and

r_(3e) designates the radius of curvature of the photographer's-eye-sidesurface of the third single lens element.

The eyepiece optical system preferably satisfies the followingcondition:

0.5<D_(b)/D_(a)<0.9  (7)

wherein

D_(a) designates the distance from the penta-mirror-side surface of thefirst single lens element to the photographer's-eye-side surface of thethird single lens element; and

D_(b) designates the distance from the penta-mirror-side surface of thesecond single lens element to the photographer's-eye-side surface of thethird single lens element which the diopter is −1.0.

The present disclosure relates to subject matters contained in JapanesePatent Application No. 10-278541 (filed on Sep. 30, 1998) and No.11-158222 (filed on Jun. 4, 1999) which are expressly incorporatedherein by reference in their entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be discussed below in detail with reference to theaccompanying drawings, in which:

FIG. 1 is a lens arrangement the first embodiment of an eyepiece opticalsystem for a single lens reflex camera, according to the presentinvention;

FIGS. 2A, 2B, 2C and 2D are aberration diagrams of the eyepiece opticalsystem shown in FIG. 1 when the diopter is −1.0;

FIGS. 3A, 3B, 3C and 3D are aberration diagrams of the eyepiece opticalsystem shown in FIG. 1 when the diopter is −2.0;

FIGS. 4A, 4B, 4C and 4D are aberration diagrams of the eyepiece opticalsystem shown in FIG. 1 when the diopter is +1.0;

FIG. 5 is a lens arrangement of the second embodiment of an eyepieceoptical system for a single lens reflex camera, according to the presentinvention;

FIGS. 6A, 6B, 6C and 6D are aberration diagrams of the eyepiece opticalsystem shown in FIG. 5 when the diopter is −1.0;

FIGS. 7A, 7B, 7C and 7D are aberration diagrams of the eyepiece opticalsystem shown in FIG. 5 when the diopter is −2.0;

FIGS. 8A, 8B, 8C and 8D are aberration diagrams of the eyepiece opticalsystem shown in FIG. 5 when the diopter is +1.0;

FIG. 9 is a lens arrangement of the third embodiment of an eyepieceoptical system for a single lens reflex camera, according to the presentinvention;

FIGS. 10A, 10B, 10C and 10D are aberration diagrams of the eyepieceoptical system shown in FIG. 9 when the diopter is −1.0;

FIGS. 11A, 11B, 11C and 11D are aberration diagrams of the eyepieceoptical system shown in FIG. 9 when the diopter is −2.0;

FIGS. 12A, 12B, 12C and 12D are aberration diagrams of the eyepieceoptical system shown in FIG. 9 when the diopter is +1.0;

FIG. 13 is a lens arrangement of the fourth embodiment of an eyepieceoptical system for a single lens reflex camera, according to the presentinvention;

FIGS. 14A, 14B, 14C and 14D are aberration diagrams of the eyepieceoptical system shown in FIG. 13 when the diopter is −1.0;

FIGS. 15A, 15B, 15C and 15D are aberration diagrams of the eyepieceoptical system shown in FIG. 13 when the diopter is −2.0;

FIGS. 16A, 16B, 16C and 16D are aberration diagrams of the eyepieceoptical system shown in FIG. 13 when the diopter is +1.0;

FIG. 17 is a lens arrangement of the fifth embodiment of an eyepieceoptical system for a single lens reflex camera, according to the presentinvention;

FIGS. 18A, 18B, 18C and 18D are aberration diagrams of the eyepieceoptical system shown in FIG. 17 when the diopter is −1.0;

FIGS. 19A, 19B, 19C and 19D are aberration diagrams of the eyepieceoptical system shown in FIG. 17 when the diopter is −2.0;

FIGS. 20A, 20B, 20C and 20D are aberration diagrams of the eyepieceoptical system shown in FIG. 17 when the diopter is +1.0;

FIG. 21 is a lens arrangement of the sixth embodiment of an eyepieceoptical system for a single lens reflex camera, according to the presentinvention;

FIGS. 22A, 22B, 22C and 22D are aberration diagrams of the eyepieceoptical system shown in FIG. 21 when the diopter is −1.0;

FIGS. 23A, 23B, 23C and 23D are aberration diagrams of the eyepieceoptical system shown in FIG. 21 when the diopter is −2.0;

FIGS. 24A, 24B, 24C and 24D are aberration diagrams of the eyepieceoptical system shown in FIG. 21 when the diopter is +1.0;

FIG. 25 is a conceptual view of a single lens reflex camera to which thepresent invention is applied; and

FIG. 26 is a lens arrangement of the eyepiece optical system of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 25 is a conceptual view of a single lens reflex camera which has apenta mirror 15 as an erecting optical system. According to the drawing,an object image formed by a photographic lens system 11 is reflected bythe quick-return mirror 12, and is formed as a real image on a focusingplate 13. The photographer views the magnified real image through theeyepiece optical system 14 and the penta mirror 15.

The embodiments relate to the structure of the eyepiece optical system14. As shown in FIG. 26, the eyepiece optical system includes a negativefirst single lens element L1, a positive second single lens element L2,and a third single lens element L3 which is constituted by a negativemeniscus lens element having a convex surface on the penta-mirror-side,in this order from the penta mirror 15. Furthermore, the second singlelens element L2 is provided as a diopter adjusting lens element, whichis adjustable along the optical axis direction. By moving the secondsingle lens element L2 towards the first single lens element L1, thediopter can be adjusted in the negative direction. On the other hand,the diopter can be adjusted in the positive direction by moving thesecond single lens element L2 towards the third single lens element L3.The advantages of such an arrangement are described hereinafter.

The penta mirror (pentagonal roof mirror) 15, known in the art, is amirror having roof-shaped reflection mirror surfaces which replace thereflection surfaces of a penta prism. The inner surfaces of theroof-shaped reflection mirror surfaces are treated with a reflectivecoating in order to form each reflection surface thereof. Since a platewhich is treated with a reflective coating is further provided on theouter side of the reflection surfaces, the contour of the penta mirror15 is made larger. Furthermore, in the case where the penta mirror 15 isused as an erecting optical system, since the reflection-optical-path isin the air, the equivalent air thickness along the optical axis cannotbe made shorter up to 1/n times of the equivalent air thickness of thepenta prism (n: the refractive index of a penta prism to be compared).Conversely, the reflection-optical-path is made longer, so that thefocal length of an eyepiece optical system is made longer, and a findermagnification is lowered.

In order to miniaturize the camera, or in order to increase themagnification of the finder, the penta mirror 15 has to be miniaturized.However, if the penta mirror 15 is miniaturized, the ‘window’ throughwhich light rays are emitted from the penta mirror 15 towards theeyepiece optical system is decreased in size, whereby it becomesdifficult to maintain a sufficient eye relief with respect to the lightrays from the periphery of the field-of-view. In particular, in a finderfor a camera in which the 135-type film being most popularized is used,if a penta mirror and a diopter adjusting function are provided, it islearnt that a conventional lens arrangement cannot maintain a sufficienteye relief over the entire area of a sufficient diopter adjustmentrange.

On the other hand, according to the embodiments, by providing thenegative first single lens element L1 which has diverging power and ispositioned at the most penta-mirror-side of the eyepiece optical system,light rays emitted from the narrow window of the penta mirror 15 is madehigher (away from the optical axis), whereby it is easier to maintain asufficient eye relief with respect to the light rays from the peripheryof the field-of-view.

Finder optical systems for single lens reflex cameras, including thoseof the embodiments, generally have positive power as a whole. In theembodiments, the first single lens element L1 has relatively strongnegative power, and light rays from the periphery of the field-of-vieware made approximately parallel to the optical axis and are then madeincident on the positive second single lens element L2 which is thediopter adjusting lens element. Due to this arrangement, even if theposition of the second single lens element L2 is varied, the change inheight of light rays incident on the second single lens element L2 issubstantially eliminated, so that light rays can be emitted from thesecond single lens element L2 at a constant emission angle regardless ofthe position of the second single lens element L2. Since the power ofthe subsequent third single lens element L3 is weaker, more concretely,the power thereof is not so strong as to change the angle of light raysfrom the second single lens element L2 in accordance with the positionof the second single lens element L2 (depending on the diopter) althoughthe third single lens element L3 can slightly bend light rays outward,through which a sufficient eye relief with respect to light rays fromthe periphery of the field-of-view can be maintained. Due to the abovepower distribution, the change in the apparent visual angle upon adiopter adjustment can be reduced.

It is also theoretically possible to design the first single lenselement L1 or the third single lens element L3 as a diopter adjustmentlens element which is movable in the optical axis direction. However,preferably, the first single lens element L1 is fixedly positionedimmediately behind the penta mirror 15 in order to increase the heightof light rays. Likewise, preferably, the third single lens element L3 isfixedly positioned since the third single lens element L3 is the finallens element which would possibly be touched by the hand of aphotographer unless a separate cover-glass is provided. Accordingly, inthe embodiments, the second single lens element L2 is used as thediopter adjusting lens element.

In order to increase the magnification of the finder, the focal lengthof the entire eyepiece optical system is made shorter as much aspossible. In other words, the principal points of the entire eyepieceoptical system are positioned as close to the penta mirror 15 aspossible. By forming the third single lens element L3 as a negativemeniscus lens element having a convex surface on the side of the pentamirror 15, the principal points can be positioned on the side of thepenta mirror 15.

Condition (1) specifies the power of the negative first single lenselement L1. If f₁/f_(e) exceeds the upper limit of condition (1), thenegative power of the first single lens element L1 becomes too strong,and light rays are diverged too much, so that the diameters of thesubsequent lens elements have to be increased. If f₁/f_(e) exceeds thelower limit of condition (1), the negative power of the first singlelens element L1 becomes too weak, so that an eye relief is notsufficiently maintained, and the negative power of the third single lenselement L3 which is given negative power has to be increased.Accordingly, condition (3) is not satisfied.

Condition (2) specifies the power of the positive second single lenselement L2 which is the diopter adjusting lens element. If the positivepower of the second single lens element L2 becomes strong to the extentthat f₂/f_(e) exceeds the lower limit of condition (2), the diopter canbe adjusted by a small amount of movement thereof; however,deterioration in aberrations due to decentration and unstableness of theimage when the positive second single lens element L2 is being movedbecome apparent. If f₂/f_(e) exceeds the upper limit of condition (2),the power of the second single lens element L2 becomes too weak. As aresult, even when the second single lens element L2 is moved along theoptical axis, the change in diopter is small, and the diopter adjustmentrange is made narrower.

Condition (3) specifies the power of the negative third single lenselement L3. If f₃/f_(e) exceeds the upper limit of condition (3), thepower of the third single lens element L3 becomes too strong, so thatthe change in the apparent visual angle upon diopter adjustment isincreased. If f₃/f_(e)exceeds the lower limit of condition (3), asufficient eye relief cannot be maintained.

Condition (4) specifies the thickness of the third single lens elementL3, whereby a sufficient eye relief is maintained while the eyepieceoptical system can be miniaturized. If d₃/(n₃×f_(e)) exceeds the upperlimit of condition (4), it is advantageous to maintain a long eyerelief; however, the size of the finder increases. If d₃/(n₃/f_(e)exceeds the lower limit of condition (4), the final lens surface of theeyepiece optical system (the photographer's-eye-side surface of thethird single lens element L3) becomes caved-in with respect to the rearsurface of the camera body, and a sufficient eye relief cannot bemaintained.

Condition (5) specifies the shape of the first single lens element L1.If SF1 exceeds the lower limit of condition (5), the first single lenselement L1 can still maintain the shape of a meniscus lens element;however, if the first single lens element L1 is made to have power whichsatisfies condition (1), the radius of curvature r_(1s) and r_(1e) ofthe front and rear lens surfaces the first single lens element L1 becomesmaller. Accordingly, if the peripheral portions of the first singlelens element L1 (on the penta-mirror side surface) and the emissionsurface of the penta mirror 15 are positioned not to interfere with eachother, the first surface of the first single lens element L1 has to bepositioned away from the penta mirror 15. As a result, the size of theeyepiece optical system increases. If SF1 exceeds the upper limit ofcondition (5), the radius of curvature of the photographer's-eye-sidesurface of the first single lens element L1 becomes smaller, and theedge portion of the first single lens element L1 approaches the secondsingle lens element L2, whereby the amount of movement of the secondsingle lens element L2 is restricted; as a result, a sufficient diopteradjustment cannot be performed.

Condition (6) specifies the shape of the negative third single lenselement L3. This condition specifies that the third single lens elementL3 is a most suitable negative meniscus lens element. If SF3 exceeds thelower limit of condition (6), the radii of curvatures of the front andrear surfaces of the third single lens element L3 increase. If the thirdsingle lens element L3 is made to have power which satisfies condition(3), the distance between the peripheral portions of the second andthird single lens elements L2 and L3 has to be decreased. However, byconsidering the structure of the frame for containing the lens elements,it is difficult for the second and third single lenses L2 and L3 toapproach each other closely. As a result, a sufficient range for diopteradjustment cannot be secured. If SF3 exceeds the upper limit ofcondition (6), the radii of curvatures of the front and rear surfaces ofthe third single lens element L3 have to be decreased in order toprovide a power which satisfies condition (3). As a result, a sufficienteye relief cannot be maintained.

Condition (7) specifies the ratio of the length of the entire eyepieceoptical system to the distance from the penta-mirror-side surface of thesecond single lens element L2 to the photographer's-eye-side surface ofthe third single lens element L3.

If D_(b)/D_(a) exceeds the lower limit of condition (7), incidentpositions of the light rays on the second single lens element L2 becometoo high, so that either the lens diameter increases, or the findermagnification is lowered. If D_(b)/D_(a) exceeds the upper limit ofcondition (7), the first and second single lens elements L1 and L2approach each other closely, and the movement of the second single lenselement L2 towards the penta mirror 12 is restricted. As a result, asufficient diopter adjustment cannot be performed.

Specific numerical examples will be herein discussed. In the tables anddiagrams, DP designates the diopter; f_(e) designates the focal lengthof the eyepiece optical system; β designates the (half amount) apparentvisual angle; θ designates the diameter of the exit pupil; h designatesthe farthest distance from a supposed optical axis on the focusing platewhen the finder coverage of the frame size for the 135-type film is 92%((21.6 (half amount)×0.92=19.9 mm); d0 designates the distance from theimage forming plane (focusing plate) of the photographing optical systemto the penta-prism-side surface of the first lens element of theeyepiece optical system; and ER designates the eye relief (the distancefrom the final surface, at the side of a photographer's eye, of theeyepiece optical system to the eyepoint (the position of the exit pupilof the finder optical system)). In the diagrams of chromatic aberrationrepresented by spherical aberrations, the solid lines and the two typesof dotted lines respectively indicate spherical aberrations with respectto the d, g and C lines. Also, in the diagrams of lateral chromaticaberration, the solid lines and the two types of dotted linesrespectively indicate magnification with respect to the d, g and Clines. S designates the sagittal image, and M designates the meridionalimage. Furthermore, R designates the radius of curvature, D designatesthe lens thickness or space between lens elements, Nd designates therefractive index of the d-line, and vd designates the Abbe number.

In addition to the above, an aspherical surface which is symmetricalwith respect to the optical axis is defined as follows:

x=Ch²/{1+[1−(1+K)C²h²]^(½)}+A4h⁴+A6h⁶+A8h⁸+A10h¹⁰;

wherein:

x designates a distance from a tangent plane of an aspherical vertex;

C designates a curvature of the aspherical vertex (1/R);

h designates a distance from the optical axis;

K designates the conic coefficient; and

A4 designates a fourth-order aspherical coefficient;

A6 designates a sixth-order aspherical coefficient;

A8 designates a eighth-order aspherical coefficient;

Embodiment 1

FIG. 1 is a lens arrangement the first embodiment of an eyepiece-opticalsystem for a single lens reflex camera, and Table 1 show the numericaldate thereof. FIGS. 2A, 2B, 2C and 2D are aberration diagrams of theeyepiece optical system shown in FIG. 1 when the diopter is −1.0. FIGS.3A, 3B, 3C and 3D are aberration diagrams of the eyepiece optical systemshown in FIG. 1 when the diopter is −2.0. FIGS. 4A, 4B, 4C and 4D areaberration diagrams of the eyepiece optical system shown in FIG. 1 whenthe diopter is +1.0. The lens arrangement in the first embodiment is thesame as the one shown in FIG. 26.

TABLE 1 DP = −1.0˜−2.0˜+1.0 (diopter) fe = 82.00˜85.52˜75.53 β =14.9˜14.6˜14.1 (degree) d0 = 77.72 ER = 15.23 No. R D Nd νd 1* −28.5402.000 1.58547 29.9 2* 241.887 1.479˜0.676˜3.215 — — 3  20.740 4.0211.49176 57.4 4* −28.888 2.926˜3.729˜1.190 — — 5  28.394 5.500 1.5854729.9 6* 17.613 — — — *designates the aspherical surface which isrotationally symmetrical with respect to the optical axis.

Aspherical surface data (the aspherical surface coefficients notindicated are zero (0.00)):

No. 1 K = 0.0 A4 = 0.2228 × 10⁻⁴ A6 = 0.6083 × 10⁻⁷ No. 2 K = 0.0 A4 =0.2090 × 10⁻⁴ A6 = 0.0 No. 4 K = 0.0 A4 = 0.3980 × 10⁻⁴ A6 = −0.9939 ×10⁻⁸ No. 6 K = 0.0 A4 = −0.8289 × 10⁻⁵ A6 = 0.1429 × 10⁻⁶

Embodiment 2

FIG. 5 is a lens arrangement of the second embodiment of an eyepieceoptical system for a single lens reflex camera, and Table 2 shows thenumerical data thereof. FIGS. 6A, 6B, 6C and 6D are aberration diagramsof the eyepiece optical system shown in FIG. 5 when the diopter is −1.0.FIGS. 7A, 7B, 7C and 7D are aberration diagrams of the eyepiece opticalsystem shown in FIG. 5 when the diopter is −2.0. FIGS. 8A, 8B, 8C and 8Dare aberration diagrams of the eyepiece optical system shown in FIG. 5when the diopter is +1.0. The lens arrangement in the second embodimentis the same as the one shown in FIG. 26.

TABLE 2 DP = −1.0˜−2.0˜+1.0 (diopter) fe = 82.00˜85.12˜76.21 β =15.0˜14.7˜14.0 (degree) d0 = 77.68 ER = 15.23 No. R D Nd νd 1* −29.7672.000 1.58547 29.9 2* 293.051 1.401˜0.508˜3.370 — — 3  20.337 4.0561.49176 57.4 4* −28.722 3.265˜4.158˜1.296 — — 5* 29.331 4.500 1.5854729.9 6* 17.345 — — — *designates the aspherical surface which isrotationally symmetrical with respect to the optical axis.

Aspherical surface data (the aspherical surface 5 coefficients notindicated are zero (0.00)):

No. 1 K = 0.0 A4 = 0.1941 × 10⁻⁴ A6 = 0.6138 × 10⁻⁷ No. 2 K = 0.0 A4 =0.1926 × 10⁻⁴ A6 = 0.0 No. 4 K = 0.0 A4 = 0.4206 × 10⁻⁴ A6 = −0.1605 ×10⁻⁷ No. 5 K = 0.0 A4 = −0.2541 × 10⁻⁵ A6 = 0.0 No. 6 K = 0.0 A4 =−0.1444 × 10⁻⁴ A6 = 0.1427 × 10⁻⁶

Embodiment 3

FIG. 9 is a lens arrangement of the third embodiment of an eyepieceoptical system for a single lens reflex camera, and Table 3 shows thenumerical data thereof. FIGS. 10A, 10B, 10C and 10D are aberrationdiagrams of the eyepiece optical system shown in FIG. 9 when the diopteris −1.0. FIGS. 11A, 11B, 11C and 11D are aberration diagrams of theeyepiece optical system shown in FIG. 9 when the diopter is −2.0. FIGS.12A, 12B, 12C and 12D are aberration diagrams of the eyepiece opticalsystem shown in FIG. 9 when the diopter is +1.0. The lens arrangement inthe third embodiment is the same as the one shown in FIG. 26.

TABLE 3 DP = −1.0˜−2.0˜+1.0 (diopter) fe = 82.00˜86.17˜74.66 β =14.5˜14.6˜14.3 (degree) d0 = 77.66 ER = 15.23 No. R D Nd νd 1* −32.3882.000 1.58547 29.9 2  154.254 1.320˜0.500˜3.001 — — 3  28.700 3.3481.49176 57.4 4* −29.530 2.791˜3.611˜1.110 — — 5  18.611 5.500 1.5854729.9 6* 15.000 — — — *designates the aspherical surface which isrotationally symmetrical with respect to the optical axis.

Aspherical surface data (the aspherical surface coefficients notindicated are zero (0.00)):

No. 1 K = 0.0 A4 = 0.7501 × 10⁻⁵ A6 = 0.0 No. 4 K = 0.0 A4 = 0.3089 ×10⁻⁴ A6 = −0.4601 × 10⁻⁷ No. 6 K = 0.0 A4 = −0.1169 × 10⁻⁴ A6 = 0.1414 ×10⁻⁶

Embodiment 4

FIG. 13 is a lens arrangement of the fourth embodiment of an eyepieceoptical system for a single lens reflex camera, and Table 4 shows thenumerical data thereof. FIGS. 14A, 14B, 14C and 14D are aberrationdiagrams of the eyepiece optical system shown in FIG. 13 when thediopter is −1.0. FIGS. 15A, 15B, 15C and 15D are aberration diagrams ofthe eyepiece optical system shown in FIG. 13 when the diopter is −2.0.FIGS. 16A, 16B, 16C and 16D are aberration diagrams of the eyepieceoptical system shown in FIG. 13 when the diopter is +1.0. The lensarrangement in the fourth embodiment is the same as the one shown inFIG. 26.

TABLE 4 DP = −1.0˜−2.0˜+1.0 (diopter) fe = 82.13˜86.26˜74.85 β =14.5˜14.6˜14.3 (degree) d0 = 77.86 ER = 15.23 No. R D Nd νd 1* −35.9012.000 1.58547 29.9 2  131.050 1.343˜0.470˜3.131 — — 3  28.700 3.5601.49176 57.4 4* −32.211 2.516˜3.389˜0.728 — — 5  18.330 5.500 1.5854729.9 6* 15.000 — — — *designates the aspherical surface which isrotationally symmetrical with respect to the optical axis.

Aspherical surface data (the aspherical surface coefficients notindicated are zero (0.00)):

No. 1 K = 0.0 A4 = 0.6200 × 10⁻⁵ A6 = 0.0 No. 4 K = 0.0 A4 = 0.2600 ×10⁻⁴ A6 = −0.3400 × 10⁻⁷ No. 6 K = 0.0 A4 = −0.5700 × 10⁻⁵ A6 = 0.7700 ×10⁻⁷

Embodiment 5

FIG. 17 is a lens arrangement of the fifth embodiment of an eyepieceoptical system for a single lens reflex camera, and Table 5 shows thenumerical data thereof. FIGS. 18A, 18B, 18C and 18D are aberrationdiagrams of the eyepiece optical system shown in FIG. 17 when thediopter is −1.0. FIGS. 19A, 19B, 19C and 19D are aberration diagrams ofthe eyepiece optical system shown in FIG. 17 when the diopter is −2.0.FIGS. 20A, 20B, 20C and 20D are aberration diagrams of the eyepieceoptical system shown in FIG. 17 when the diopter is +1.0. The lensarrangement in the fifth embodiment is the same as the one shown in FIG.26.

TABLE 5 DP = −1.0˜−2.0˜+1.0 (diopter) fe = 79.20˜82.17˜73.73 β =15.0˜15.2˜14.6 (degree) d0 = 77.86 ER = 15.23 No. R D Nd νd 1* −41.5011.800 1.58547 29.9 2  96.200 1.389˜0.500˜3.291 — — 3  21.950 3.8501.49176 57.4 4* −31.137 3.035˜3.924˜1.133 — — 5  22.490 5.300 1.5854729.9 6* 15.000 — — — *designates the aspherical surface which isrotationally symmetrical with respect to the optical axis.

Aspherical surface data (the aspherical surface coefficients notindicated are zero (0.00)):

No. 1 K = 0.0 A4 = 0.4860 × 10⁻⁵ A6 = 0.3230 × 10⁻⁷ No. 4 K = 0.0 A4 =0.4550 × 10⁻⁴ A6 = −0.5200 × 10⁻⁷ No. 6 K = 0.0 A4 = −0.2870 × 10⁻⁴ A6 =0.2370 × 10⁻⁶

Embodiment 6

FIG. 21 is a lens arrangement of the sixth embodiment of an eyepieceoptical system for a single lens reflex camera, and Table 6 shows thenumerical data thereof. FIGS. 22A, 22B, 22C and 22D are aberrationdiagrams of the eyepiece optical system shown in FIG. 21 when thediopter is −1.0. FIGS. 23A, 23B, 23C and 23D are aberration diagrams ofthe eyepiece optical system shown in FIG. 21 when the diopter is −2.0.FIGS. 24A, 24B, 24C and 24D are aberration diagrams of the eyepieceoptical system shown in FIG. 21 when the diopter is +1.0. The lensarrangement in the sixth embodiment is the same as the one shown in FIG.26.

TABLE 6 DP = −1.0˜−2.0˜+1.0 (diopter) fe = 79.20˜82.00˜74.03 β =15.0˜15.2˜14.6 (degree) d0 = 77.86 ER = 15.23 No. R D Nd νd 1* −42.2741.800 1.58547 29.9 2  79.392 1.362˜0.500˜3.238 — — 3  19.975 3.8501.49176 57.4 4* −29.243 2.853˜3.715˜0.977 — — 5  25.740 5.300 1.5253856.3 6* 15.000 — — — *designates the aspherical surface which isrotationally symmetrical with respect to the optical axis.

Aspherical surface data (the aspherical surface coefficients notindicated are zero (0.00)):

No. 1 K = 0.0 A4 = 0.3700 × 10⁻⁵ A6 = 0.5920 × 10⁻⁷ No. 4 K = 0.0 A4 =0.5550 × 10⁻⁴ A6 = −0.5240 × 10⁻⁷ No. 6 K = 0.0 A4 = −0.3800 × 10⁻⁴ A6 =0.3410 × 10⁻⁶

Table 7 shows the values of each condition for each embodiment.

TABLE 7 Embodiment 1 Embodiment 2 Embodiment 3 Embodiment 4 Embodiment 5Embodiment 6 Condition (1) −0.53 −0.56 −0.56 −0.58 −0.62 −0.59 Condition(2) 0.31 0.30 0.37 0.38 0.34 0.31 Condition (3) −1.19 −1.03 −3.68 −4.40−1.31 −1.04 Condition (4) 0.042 0.035 0.042 0.042 0.042 0.044 Condition(5) −0.79 −0.82 −0.65 −0.57 −0.40 −0.31 Condition (6) 4.27 3.89 9.3110.01 5.04 3.79 Condition (7) 0.782 0.777 0.778 0.77 0.793 0.791

As clearly shown in Table 7, embodiments 1 through 6 satisfy conditions(1) through (7). Furthermore, as shown in the aberration diagrams,aberrations at each diopter have been adequately corrected.

According to the above description, an eyepiece optical system, for asingle lens reflex camera having a penta mirror, can perform a diopteradjustment, can eliminate the change in the apparent visual angle when adiopter adjustment is being performed, and can sufficiently maintain aneye relief. Furthermore, the entire structure of the eyepiece opticalsystem, including the penta mirror, can be made smaller.

What is claimed is:
 1. An eyepiece optical system for a single lensreflex camera having a penta mirror, comprising: a negative first singlelens element, a positive second single lens element, and a negativethird single lens element, in this order from said penta mirror; whereinsaid second single lens element is a diopter adjusting lens elementwhich is adjustable along the optical axis; and wherein said eyepieceoptical system satisfies the following conditions: −2<f₁/f_(e)<00.2<f₂/f_(e)<0.5 −10<f₃/f_(e)<−1 wherein f₁ designates the focal lengthof said first single lens element; f₂ designates the focal length ofsaid second single lens element; f₃ designates the focal length of saidthird single lens element; and f_(e) designates the focal length of theentire eyepiece optical system when the diopter is −1.0.
 2. The eyepieceoptical system for a single lens reflex camera according to claim 1,wherein the following condition is satisfied: 0.02<d₃/(n₃×f_(e))<0.07wherein d₃ designates the thickness of said third single lens element;and n₃ designates the refractive index of said third single lens elementwith respect to the d-line.
 3. The eyepiece optical system for a singlelens reflex camera according to claim 1, wherein the following conditionis satisfied: −1.0<SF1<0 wherein SF1 designates a value defined by(r_(1s)+r_(1e))/(r_(1s)−r_(1e)); r_(1s) designates the radius ofcurvature of the penta-mirror-side surface of said first single lenselement; and r_(1e) designates the radius of curvature of thephotographer's-eye-side surface of said first single lens element. 4.The eyepiece optical system for a single lens reflex camera according toclaim 1, wherein said third single lens element comprises a negativemeniscus lens element having a convex surface on the side of said pentamirror.
 5. The eyepiece optical system for a single lens reflex cameraaccording to claim 4, wherein the following condition is satisfied:3<SF3<12; wherein SF3 designates a value defined by(r_(3s)+r_(3e))/(r_(3s)−r_(3e)); r_(3s) designates the radius ofcurvature of the penta-mirror-side surface of said third single lenselement; and r_(3e) designates the radius of curvature of thephotographer-eye-side surface of said third single lens element.
 6. Theeyepiece optical system for a single lens reflex camera according toclaim 1, wherein the following condition is satisfied:0.5<D_(b)/D_(a)<0.9; wherein D_(a) designates the distance from thepenta-mirror-side surface of said first single lens element to thephotographer's-eye-side surface of said third single lens element; andD_(b) designates the distance from the penta-mirror-side surface of saidsecond single lens element to the photographer's-eye-side surface ofsaid third single lens element which the diopter is −1.0.